This invention relates to a leak detection system, and more particularly to a leak detection system that is capable of being employed for purposes of not only determining the occurrence of a leak in a substantially closed vessel but also for determining the probable location and/or direction of flow of the leak therewithin.
To those skilled in this field of art, it is well-known that one of the major operating components of any steam generation system is the steam generator. For, it is within the latter that the transformation of water into steam is effectuated. In this regard, such a steam generator may be viewed as comprising a substantially closed vessel within which a pair of operating fluids are made to flow such that heat is transferred from one of the pair of operating fluids to the other and without the occurrence of any commingling of the two operating fluids.
Commonly, the construction of the steam generator referred to above is such that one of the operating fluids is made to flow through a multiplicity of tubes that are provided for this purpose in the steam generator, while the other operating fluid is made to flow in surrounding relation to the aforesaid multiplicity of tubes. Moreover, in accord with conventional practice, the ends of the tubes are suitably retained within a pair of spaced tube sheets. That is, the subject steam generator is generally substantially cylindrical in configuration, and has a tube sheet suitably mounted therewithin such as to be positioned adjacent to but spaced from each of the ends of the steam generator. Each of the multiplicity of tubes in turn is suitably supported within the steam generator so as to extend longitudinally therewithin with the respective ends thereof emplaced in a corresponding one of the aforesaid pair of spaced tube sheets.
Leaks are known to occur in steam generators that embody the type of construction, which has been described above. Moreover, one source of such leaks is known to be attributable to tube failures. Another known source of these leaks is a failure in one or more of the welds that are employed for purposes of affixing the ends of the tubes to the tube sheets. For obvious reasons, it is desirable that the occurrence of such leaks be minimized. However, it is also equally desirable that the existence of such leaks be noted as soon as possible after their commencement so that, where deemed appropriate, suitable corrective measures may be immediately initiated. This is done primarily so as to minimize the extent to which the respective operating fluids become commingled.
Commingling of the two operating fluids is to be avoided for various reasons. For example, in the case of PWR applications wherein the steam generator is connected in fluid flow relation directly to a nuclear reactor, one of the two operating fluids; namely, that which is at the higher temperature is made to circulate between the nuclear reactor and the steam generator. Moreover, in the course of its passage through the nuclear reactor this operating fluid becomes exposed to radiation such that by the time it leaves the nuclear reactor and enters the steam generator it possesses radioactive contamination. It is, therefore, desirable that a commingling of this operating fluid, which possesses radioactive contamination, with the other operating fluid that is being circulated through the steam generator be avoided, so as to prevent the latter from becoming contaminated with the radiation that is possessed by the former.
Another situation wherein the commingling of the two operating fluids is to be avoided is where the latter are dissimilar. For example, it is known to employ a liquid metal as one of the operating fluids in the steam generator of a nuclear-powered steam generation system. More specifically, in such systems it has been proposed to employ a liquid metal such as sodium as one of the two operating fluids in the steam generator, with water being the other operating fluid. However, in view of the fact that an admixture of water and sodium is known to be capable of producing a violent reaction, particular consideration must be given to the avoidance of any commingling thereof when these two fluids are being utilized as the respective operating fluids in a steam generator.
Having thus set forth some of the various reasons as to why a commingling of the operating fluids is desirably to be avoided in the steam generator, attention will now be directed to a consideration of some of the potential sources of leaks which can occasion such a commingling of the two operating fluids. By way of exemplification in this regard, one potential source of leaks is a failure in the tubes through which one of the operating fluids is designed to flow while being kept isolated from the other operating fluid in terms of any commingling of fluid therebetween, but while yet permitting a transfer of heat to occur between the respective fluids. It is known in the construction of such steam generators to employ either single tubes that extend substantially the length of the steam generator as well as so-called duplex tubes. In accord with the latter form of construction, each tube consists of a pair of concentric tubes with a suitable space being formed therebetween. Obviously, in the case of the form of construction wherein a single tube is employed, should a failure in the tube occur, there would be an immediate commingling of the operating fluid flowing through the tube with the operating fluid flowing in surrounding relation to the tube which failed. The extent to which an admixture of the respective operating fluids would occur is a function of many variables including such things as, for example, the size of the leak, the relative pressures of the two operating fluids, etc. On the other hand, in the case of the so-called duplex tube, should a failure occur in either the inner tube or the outer tube thereof but not in both, it is unlikely that any commingling of the two operating fluids would take place inasmuch as the good tube, i.e., the inner tube or the outer tube which ever one did not fail, would continue to function as an isolating barrier between the operating fluid flowing in the duplex tube and the operating fluid flowing in surrounding relation to the duplex tube.
In accord with conventional practice, the tubes, be they in the form of either a single tube or a duplex tube, are commonly affixed to the tube sheets by means of welding. More specifically, the tubes are ordinarily welded to the tube sheets on either side of the latter at the locations whereat the former pass therethrough. In addition, the tubes may also be welded to the tube sheets at any other location where it may be deemed desirable to establish such a weld. With regard to the matter of potential sources of leaks, any of the aforementioned welds that are made between the tubes and tube sheets may give rise to a leak which could enable the two operating fluids to commingle. Whether or not a weld failure gives rise to the possibility of the respective operating fluids commingling obviously depends on whether the failure of the particular weld in question enables a through passage to be established between the respective flow paths of the two operating fluids. Based on experience of those skilled in this art, it has been found that the probability of the occurrence of a weld failure is deemed to be considerably more likely than the occurrence of a tube failure be the tube either a single tube, i.e., a single wall tube, or a duplex tube, i.e., a double wall tube.
Although either tube leaks or weld leaks may individually give rise to a commingling of the respective operating fluids, the existence of an admixture of operating fluids may also be caused by a combination of both tube and weld leaks. That is, a situation may arise wherein a tube leak occurs as well as a weld leak, although neither alone is sufficient to cause a commingling of the operating fluids to take place. However, the collective effect of the concomitant failure of a particular tube or tubes when combined with the failure of a particular weld or welds may be sufficient to create a through path for fluid flow from one of the operating fluids to the other and thereby cause a commingling thereof to come about.
There has been some attention directed by the prior art to this matter of the admixture of respective operating fluids in a steam generator caused by the occurrence of leaks therein that result in the establishment of a passage through which fluid may flow from one operating fluid to another. Exemplary of this attention is the teachings to be found contained in U.S. Pat. No. 3,129,697--Trepaud. Namely, as taught in the latter patent, a leak detection system is provided wherein each of a pair of tube sheets is divided into two segments, i.e., two complementary half plates. At least one of the segments of each pair thereof of each of the tube sheets is provided with a plurality of grooves that operatively connect together in series relation a number of circular recesses that are suitably located in surrounding relation to the ends of the tubes that are associated with the tube sheets. The mode of operation of the aforedescribed prior art form of leak detection system is such that should a leak develope between the tube and the corresponding surrounding portion of the tube sheet, the leaking fluid is designed to make its way into one of the circular recesses described above, and therefrom into one of the grooves also described above from whence the fluid that has leaked is caused to be discharged at a pre-selected location. No attempt is made, however, in accordance with the teachings of U.S. Pat. No. 3,129,697--Trepaud to discern the specific source of the leak. Another disadvantage associated with the use of the leak detection system as taught in the Trepaud Patent is that it is predicated on the use of complementary half plates. Accordingly, since each of the tube sheets is formed in the manner of a pair of complementary half plates, a need exists to provide a fluid-tight seal between each pair of half plates. Unfortunately, as is well-known to all, it is extremely difficult to establish a fluid-tight seal between a pair of mating surfaces. More specifically, as applied to the Trepaud Patent, it is extremely difficult to prevent fluid from migrating from one series of groove interconnected recesses across the face of the half plate to another series of groove interconnected recesses, notwithstanding how tight the half plates may have been secured together. Namely, it is well-known that the establishment of a totally fluid-tight seal between two mating surfaces that are made of metal is extremely difficult to achieve. Thus, it is not surprising that in the case of the leak detection system taught in U.S. Pat. No. 3,129,697, the isolation between grooves and/or recesses which would be required in order to provide a leak detection system that embodies the capability of enabling a determination to be had as to the probable location and/or direction of flow of a leak that has occurred is unachieveable with the structure as taught in the subject Trepaud Patent.
Therefore, notwithstanding the teachings of the prior art as exemplified, for instance, by those found contained in U.S. Pat. No. 3,129,697, a need has existed for a new and improved leak detection system. More specifically, a need has been shown for a new and improved form of leak detection system that would be particularly suited for use in a steam generator wherein it is desired to prevent the operating fluids flowing therethrough from commingling. Preventing the creation of an admixture of the respective operating fluids may be occasioned wherein both operating fluids are similar in nature by the desire to prevent one of these operating fluids from becoming contaminated by the radiation contamination possessed by the other, or wherein the operating fluids are dissimilar in nature by the desire to prevent the generation of a violent reaction occasioned by the mixing together of the two operating fluids wherein one of the latter is a liquid metal such as sodium and the other is water.
Namely, a need has been evidenced for a new and improved leak detection system that would be operable for purposes of sensing the occurrence of a leak that was caused by a tube failure, a weld failure, or a combination of tube and weld failure. Moreover, such a new and improved form of leak detection system should be operable for purposes of sensing the occurrence of a leak caused by a tube failure irrespective of whether the tube consists of a single tube, i.e., a single wall tube, or a duplex tube, i.e., a double wall tube. Furthermore, such a new and improved leak detection system should also embody the capability of enabling a determination to be had based on a sensing made thereby as to the probable location and/or direction of flow of the leak that was sensed.
It is, therefore, an object of the present invention to provide a new and improved form of leak detection system that is suitable for use in a steam generator and which is operable therein for purposes of detecting a leak that could give rise to the commingling of a pair of operating fluids that desirably are designed to flow through the steam generator in fluid isolation one to another.
It is another object of the present invention to provide such a leak detection system that is operable to detect a leak of an operating fluid in a steam generator caused by a tube failure.
It is still another object of the present invention to provide such a leak detection system which is operable to detect a leak caused by a tube failure wherein the tube is a single tube, i.e., of a single wall construction.
A further object of the present invention is to provide such a leak detection system which is operable to detect a leak caused by a tube failure wherein the tube is a duplex tube, i.e., of double wall construction.
A still further object of the present invention is to provide such a leak detection system which is operable to detect a leak of an operating fluid in a steam generator caused by a weld failure.
Yet another object of the present invention is to provide such a leak detection system that is operable to detect a leak of an operating fluid in a steam generator caused by a combination of a tube failure and a weld failure.
Yet still another object of the present invention is to provide such a leak detection system which embodies the capability of enabling a determination to be had as to the probable location and/or direction of flow of the leak that has been sensed, based on the sensing thereof that has been made thereby.